Chlorinated benzenes are conventionally produced by reacting benzene with chlorine. The products of the chlorination generally include monochlorobenzene, dichlorobenzenes and to a lesser extent, trichlorobenzenes, tetrachlorobenzenes, pentachlorobenzene and hexachlorobenzene. The product composition is governed by the inherent thermodynamic equilibria but may be controlled to some extent by varying reaction conditions (chlorine to benzene ratio, catalyst, temperature and residence time).
Separation of the individual products involves sequential distillations to recover unreacted benzene, monochlorobenzene and a mixture of dichlorobenzenes consisting of 1,2-dichlorobenzene, 1,3-dichlorobenzene and 1,4-dichlorobenzene. The 1,2-dichlorobenzene can be obtained in high purity by conventional distillation. The 1,4-dichlorobenzene, on the other hand, cannot be separated from 1,3-dichlorobenzene by conventional distillation because their respective boiling points are within 1.degree. C. of each other. Most of the 1,4-dichlorobenzene can be recovered by crystallization at sub-ambient temperatures, but complete separation cannot be achieved because of the existence of a eutectic composition. Depending on the refrigeration employed, the process purge from the crystallization step may typically contain 20 to 40 percent 1,4-dichlorobenzene and 60 to 80 percent 1,3-dichlorobenzene. This mixture is not a saleable product though each of its constituents is a valuable product. For example, 1,3-dichlorobenzene is a starting raw material in the production of several new crop-protection chemicals.
U.S. Pat. No. 4,059,642 issued to Dewald et al. on Nov. 22, 1977 discloses a process for selectively alkylating a mixture of 1,3- and 1,4-dichlorobenzenes to yield a mixture of 1,4-dichlorobenzene and 2,4-dichloroalkylbenzene, which can then be separated by conventional separation techniques. As shown in the examples, isopropyl chloride is used as the alkylating agent and the dichlorobenzenes are selectively alkylated in the presence of aluminum chloride as catalyst.
U.S. Pat. No. 3,553,274 issued to Lewis et al. on Jan. 5, 1971 shows use of an HAlBrCl.sub.3 catalyst to alkylate 1,3-dichlorobenzene to a mixture comprising 2,4-dichloroisopropyl benzene and 3,5-dichloroisopropyl benzene. As shown in Example 1, one mole of isopropyl bromide was used per mole of 1,3-dichlorobenzene. The yield of dichloroisopropyl benzene was only 70%.
U.S. Pat. No. 4,104,315 issued to Dewald et al. on Aug. 1, 1978, discloses a method of separating 3,5-dihaloalylbenzenes from 2,4-dihaloalkylbenzenes by selective alkylation. In an illustrative example, 2,4-dichloroisopropyl benzene was preferentially alkylated with isopropyl chloride to 2,4-dichloro-5-isopropylcumene. The unreacted 3,5-dichlorocumene was separated by simple distillation from the 2,4-dichloro-5-isopropyl cumene. As shown in the example, 90.5 percent of the 3,5-dichlorocumene present in the original mixture was removed. Unfortunately, the process leads to production of 2,4-dichloro-5-isopropylcumene which represents an undesirable by-product.
Finally, U.S. Pat. No. 4,329,524 issued to Dewald on May 11, 1982 shows a process for selectively transalkylating an isomeric mixture of 3,5- and 2,4-dichlorocumene, in the presence of excess benzene. This leads to a reaction product including cumene, 3,5-dichlorocumene and 1,3-dichlorobenzene.
None of the prior art shows a process for synthesizing 3,5-dichloroalkylbenzene and recovering 1,3-dichlorobenzene from a mixture of 1,3- and 1,4-dichlorobenzenes. The prior art also does not recognize the possibility of obtaining the above products from the alkylation of 1,3-dichlorobenzene, without production of undesirable by-products.